In marine seismic prospecting, an exploration vessel tows a seismic streamer having a plurality of pressure sensitive detectors, commonly referred to as hydrophones. A source of seismic energy, such as an air gun or an explosive charge, is used to propagate pressure waves through the water into the underlying sea floor. Part of the energy will be reflected by subfloor geological discontinuities and subsequently detected by the hydrophones as pressure variations in the surrounding water. The mechanical energy of these pressure variations is transformed into an electrical signal by the hydrophones and transmitted through the streamer to a recording apparatus aboard the vessel. The collected data may then be interpreted by those skilled in the art to reveal information about the subsea geological formations. Modern methods of seismic data collection use redundant subsurface coverage by successive source-hydrophone positions. This allows the data to be stacked to enhance the signal to noise ratio.
For the signals to be meaningful and to permit correct stacking of signals from different source and hydrophone positions, it is necessary to know the placement of the individual hydrophones at the time the pressure waves are detected. As the vessel is continuously moving and as the streamer may extend for thousands of feet behind the vessel, accurate location of the streamer hydrophones relative to fixed positions at the underwater bottom is difficult.
Various systems have been developed to provide accurate information as to the location of the vessel. In a common application a plurality of underwater transponders generate unique output frequency signals in response to an interrogation signal from the ship. The transit time for the interrogation signal and the transponder's response signal is measured and the distance or range from each transponder is calculated. The vessel's position with respect to the transponders may then be triangulated if the location of the transponders are known.
It has also been proposed to measure the distance from an underwater vessel from a tow vessel using the plural transponders and an additional transponder in the underwater vessel. However, none of the signals receive from any of the transponders indicate relative velocity between the bottom positioned transponders and either the towed vessel or any of a plurality of locations along the towed cable.
However, it is rare for the streamer to trail directly along the path of the vessel. While the streamer is attached to the stern of the vessel, the bulk of the streamer is submerged below the water surface through the action of depth controllers along the length of the streamer. As a result, the cross-track current at the streamer depth may differ from the cross-track current affecting the vessel, thereby causing the streamer to trail at an angle to the vessel's course. Other factors, which are not necessary to enumerate, may also create a variance in the path of the streamer when compared to the vessel track.
One method of estimating the location of the streamer disclosed in the prior art relies upon the addition of a tail buoy radar reflector located at the end of the streamer. On-board radar systems may then be used under optimal sea conditions to find the end of the streamer and the location of the individual hydrophones interpolated. Such systems are generally unreliable however, and render the required data suspect.
A second method taught by the art relies upon very sensitive and expensive apparatus to measure the yaw and pitch angles of the streamer end adjacent the vessel. These data, coupled with magnetic compass headings taken along the streamer and the known depth of the streamer, permits one to empirically calculate the hydrophone locations.
It is an object of this invention to provide an accurate, alternative means for locating the submerged streamer relative to known underwater locations which adequately account for relative motion between the streamer and such locations during traverse of a seismic line with the streamer.